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25 Sep 2013 00:42 Age: 204 days
Category: Nomenclature and Terminology

Standard Atomic Weights Revised

24 SEPTEMBER 2013 - The IUPAC Commission on Isotopic Abundances and Atomic Weights (II.1) met under the chairmanship of Dr. Willi A. Brand, at the Scientific and Technological Research Council of Turkey in Gebze, prior to the IUPAC General Assembly in Istanbul, Turkey, in August 2013. Following its meeting, the Commission recommended changes to the standard atomic weights of 19 chemical elements. The IUPAC Council, at its meeting on 14-15 August, approved these changes. The changes are the result of cooperative research supported by the U.S. Geological Survey, IUPAC, and other contributing Commission members and institutions. 

The standard atomic weights of cadmium, molybdenum, selenium, and thorium have been changed based on recent determinations of terrestrial isotopic abundances. In addition, the standard atomic weights of 15 elements have been revised based on the new assessment of their atomic masses by International Union of Pure and Applied Physics.

The following changes in the standard atomic weights have been made:

aluminium (aluminum): from 26.981 5386(8) to 26.981 5385(7)

arsenic: from 74.921 60(2) to 74.921 595(6)

beryllium: from 9.012 182(3) to 9.012 1831(5)

cadmium: from 112.411(8) to 112.414(4)

caesium (cesium): from 132.905 4519(2) to 132.905 451 96(6)

cobalt: from 58.933 195(5) to 58.933 194(4)

fluorine: from 18.998 4032(5) to 18.998 403 163(6)

gold: from 196.966 569(4) to 196.966 569(5)

holmium: from 164.930 32(2) to 164.930 33(2)

manganese: from 54.938 045(5) to 54.938 044(3)

molybdenum: from 95.96(2) to 95.95(1)

niobium: from 92.906 38(2) to 92.906 37(2)

phosphorus: from 30.973 762(2) to 30.973 761 998(5)

praseodymium: from 140.907 65(2) to 140.907 66(2)

scandium: from 44.955 912(6) to 44.955 908(5)

selenium: from 78.96(3) to 78.971(8)

thorium: from 232.038 06(2) to 232.0377(4)

thulium: from 168.934 21(2) to 168.934 22(2)

yttrium: from 88.905 85(2) to 88.905 84(2)

These changes in the standard atomic weights will be published in a new “Table of Standard Atomic Weights 2013”, which will be published in Pure and Applied Chemistry in 2014. The revised values for atomic weights can be found on-line at the website of the Commission on Isotopic Abundances and Atomic Weights (www.ciaaw.org).

The Commission has also resolved to reclassify thorium from a mono- to a bi-isotopic element owing to the significant abundance of the thorium-230 isotope in deep seawaters. The Commission also recommended a new standard value for the isotope ratio of uranium, N(238U)/N(235U) = 137.8(1), in naturally occurring terrerestrial materials.

The importance of determining precise atomic weights has long been recognized, resulting in the creation of the International Atomic Weights Committee in 1899. IUPAC has overseen the periodic evaluation and dissemination of standard atomic-weight values since its formation in 1919.


Dr. Juris Meija <juris.meija@nrc.ca>,
Secretary of IUPAC Commission II.1